1. Field of the Invention
The invention concerns a device for correcting the faults of a sequence of images analyzed by an integrating matrix infrared sensor, said sensor being formed by a matrix of photosensitive cells giving a video signal that successively expresses the integrated luminance of each pixel of an image.
Standard infrared cameras include an individual detector or a linear array of detectors analyzing an image by means of an optomechanical scanning operation. They give a video signal expressing the instantaneous luminance of each pixel. There now exist matrix sensors that do not call for any optomechanical scanning to analyze an image. The successive analysis of the pixels is done inside the sensor, by charge-coupled or charge-transfer or other types of electronic devices. In any case, the sensor includes: zones for storing the charges generated by the photons, between two readings, an addressing device enabling access selectively to the storage zones and a charge-transfer device enabling the removal of the charges towards an output and the obtaining of a video signal successively expressing the quantity of the charges stored in each of the storage zones.
The quantity of charges stored in a storage zone is a function of the luminance of the pixel corresponding to this zone and is proportionate to the time between two readings. The corresponding vide signal therefore represents the integrated luminance. When an image has a background having a certain degree of luminance, the video signal includes a continuous component that is a function of the luminance of the image background. This luminance is itself a function of the temperature of the background, since the images are infra-red ones. This continuous component has a very high relative value with respect to the variable component representing a scene. In the spectral bands extending from 3 to 5 micrometers and from 8 to 12 micrometers for example, a temperature difference equal to one degree, between the scene and the background, is expressed typically by a variation of 1% between the video signal and the continuous component.
To exploit the video signal, it is necessary to eliminate the continuous component in order to amplify only the variable component corresponding to the details of the scene. Unfortunately, it is not possible to subtract simply a constant value from the video signal, firstly because each cell of the sensor gives a slightly different response from that of the other cells, and secondly because this response varies as a function of the luminance of the image background, that is, it varies as a function of the temperature of this image background, and secondarily as a function of the temperature of the structures surrounding the sensor.
During the analysis of an image representing a uniform background, the cells of one and the same sensor give slightly different responses which constitute a video signal comprising a fixed noise superimposed on a continuous component equal to the mean response of the cells. The values of sensitivity of the cells of one and the same sensor are distributed, roughly, according to a Gaussian relationship having a standard deviation equal to a few percent. When there is no correction, a sequence of images representing a uniform background is restored by images having constant faults of uniformity. These faults of uniformity are troublesome not only for the observation of the restored images but also for an operation such as a target detection or a target tracking operation. To correct the restored images, it is necessary to correct the video signal in such a way that it has a constant level for a uniform background, and that this level is maintained when the temperature of the image background changes.
2. Description of the Prior Art
A first known method for correcting such images consists simply in memorizing the values of the video signal of an image having a uniform background, during a period of calibration, then in subtracting the values of the video signal of this image respectively from the values of the video signal of the current images. For the calibration, an image representing a uniform background is obtained by placing a shutter before the objective of the camera, or else by defocusing the images of the current scene.
This first correction method makes it possible to cancel precisely the video signal corresponding to the uniform background, for the temperature which was that of the background at the instant of the calibration. When the temperature of the background changes, the cancellation is no longer done. Not only does a response in variation appear for each cell but, in addition, this variation is not identical despite the uniformity of the background, since the sensitivity of the cells is not uniform.
A second known method consists in making, in addition to this fixed correction, a correction that is variable as a function of the temperature of the background and a correction that is variable as a function of the temperature of the structures surrounding the sensor. Indeed, the sensor sees not only the image to be analyzed but also infra-red rays emitted by the structures surrounding it. The second method consists, therefore, in making a measurement in the laboratory, once and for all, of the sensitivity of each cell with respect to the temperature of the background and the sensitivity of each cell with respect to the temperature of the structures, and in storing these values of sensitivity in two memories. These values of sensitivity are exact for a given temperature, and are only approximate in the neighborhood of this temperature. To correct a sequence of images, the method then consists in: measuring the temperature of the background of the images and the temperature of the structures, computing two correction signals, for each pixel, as a function of these measurements of temperature and as a function of the values of sensitivity stored in memory, then in subtracting these two variable correction signals from the video signal that has already undergone the subtraction of a fixed correction signal determined according to the first known method.
This second known method has the drawback of being complicated to implement since it makes it necessary to measure the temperature of the background and the temperature of the structure, and calls for high precision because the computation error on all the correction signals should remain far below the values of the signal representing the scene which, it may be recalled, are of the order of 1% of the value of the continuous component of the video signal before correction. Moreover, it is not possible to modify the values of sensitivity stored in memory, for they must be measured in the laboratory to make it possible to cause variation in both the temperatures. When the background temperature varies greatly, the stored values no longer enable a sufficiently precise correction to be made. Faults appear in the restored images.